Coulometry is an electrochemical technique that measures the total coulombs of electricity consumed or produced during electrochemical reactions. Today, there are two basic categories of coulometric techniques. Controlled-potential coulometry applies a constant potential to the electrochemical cell using a potentiostat. Controlled-current coulometry applies a constant current to the electrochemical cell using a galvanostat. In both cases, the total charge, Q, passed through the electrochemical cells is calculated by integrating the current as a function of time. Coulometry has many applications in the field of electrochemistry. For example, coulometry can be used to characterize the performance of batteries, fuel cells, or other electrochemical reactions.
Lithium-ion (Li-ion) batteries are becoming a dominant battery chemistry. A Li-ion battery is a type of rechargeable battery in which lithium ions move from the negative electrode to the positive electrode during discharge and back when charging. Due primarily to their high energy density, Li-ion batteries have become the most common battery type in modern battery electric vehicles (BEVs). Unlike the starting, lighting, and ignition batteries, the Li-ion batteries in BEVs supply power over sustained periods of time, and are characterized by their relatively high power-to-weight ratio, specific energy and energy density. They also have low self-discharge rates.
Cycle life is a measure of the number of charge-discharge cycles that a battery can perform before its nominal capacity falls below a specific threshold (e.g., 80% of its initial rated capacity). Cycle life varies tremendously depending on cycling conditions, chemistry, cell design, and manufacturing quality. For example, depending on how a Li-ion battery is used in application, the Li-ion battery may present significant degradation lasting only hundreds of cycles or could be designed to last for many thousand cycles.
Technologies that improve abuse tolerance or cycle life of Li-ion batteries would have large commercial interest. However, proving new technologies or even iterating on existing formulations to improve cycle life is challenging given that Li-ion chemistries exist that can cycle for many years.
Cycle testing is an important battery qualification test. A common battery evaluation process entails cycling a test cell under a fixed current between a lower and upper voltage limit to study the performance of the batteries over the lifetime of the cycling. Cells are subjected to repeated charge-discharge cycles to verify that the cells meet the manufacturer's claimed cycle life. Various cell performance parameters such as temperature, capacity, impedance, power output and discharge time can be monitored and recorded during the test. The cycle testing verifies that the battery performance is in-line with the end product reliability and lifetime expectations.
High precision coulometers are capable of measuring the coulombic efficiency of a Li-ion battery to 50 ppm accuracy. The coulombic efficiency metric compares the coulombs required to charge a battery to the coulombs obtained from discharge. Additional battery-specific tests can also be conducted under stringent and a broad range of test conditions to assess risks associated with the battery under test. Knowledge of cell performance under various operating environments and usage patterns can be helpful to optimize battery design for various applications.